Load mentionValues asynchronously (#36739)

Eliminate a few database queries on all issue and pull request pages by
moving mention autocomplete data to async JSON endpoints fetched
on-demand when the user types `@`.

See https://github.com/go-gitea/gitea/pull/36739#issuecomment-3963184858
for the full table of affected pages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/issue_page_meta.go

@@ -155,8 +155,7 @@ func (d *IssuePageMetaData) retrieveAssigneesData(ctx *context.Context) {
 		}
 		d.AssigneesData.SelectedAssigneeIDs = strings.Join(ids, ",")
 	}
-	// FIXME: this is a tricky part which writes ctx.Data["Mentionable*"]
-	handleMentionableAssigneesAndTeams(ctx, d.AssigneesData.CandidateAssignees)
+	ctx.Data["Assignees"] = d.AssigneesData.CandidateAssignees
 }
 
 func (d *IssuePageMetaData) retrieveProjectsDataForIssueWriter(ctx *context.Context) {